A container made of polyester resin such as PET (polyethylene terephthalate) resin is typically a bottle, a cup or a tray, and has excellent transparency, impact resistance, gas impermeability and so on. It is produced and used in a lot of kinds and shapes, thus the production of it is one of the most important in industrial field. For example, a bottle included in polyester resin containers is produced as follows; heating a preform prepared by injection molding to a temperature range of 100 to 130° C. which is higher than the glass transition temperature (Tg); and biaxial stretch blow molding the preform in a metal mold for forming shortly thereafter.
Also, a heat resistant polyester resin bottle used for hot packing is provided with heat resistance by heating a metal mold for forming to a crystallizing temperature range of polyester resin, that is 100 to 180° C., in order to heat set the bottle in a high temperature range after the biaxial stretch blow molding.
On the other hand, cup or tray included in polyester resin container is made of a polyester sheet by vacuum forming or pressure forming in a female mold (metal mold for forming) having a same shape as the container, drawing by the female mold and a plug, or made of a preform by biaxial stretch blow molding by using a plug and a female mold in the same manner as the production of above-mentioned bottle.
Also, a heat resistant cup or tray used for hot packing is provided with heat resistance by heating a female mold and a plug to a crystallizing temperature range of polyester resin for forming as with the case of the bottle.
Since a metal mold for forming such polyester resin containers needs to have releasability with respect to polyester resin during forming as well as smoothness of inner surface of the metal mold for the purpose of providing the product with surface luster and transparency, a mold made of stainless steel or aluminum alloy, the surface thereof being mirror finished after the inner surface thereof being machined has been used as a metal mold for forming.
Also, in the case of forming a heat resistant polyester resin container, there has been a problem that a part of polyester resin on the surface of container, oligomer ingredient in the polyester resin or additive adheres and builds up on the surface of metal mold, and then the configuration of the buildup transfers onto the surface of the polyester resin container after forming, which adhersely effects luster and transparency of the container, so the metal mold has to be cleaned in a short period of time.
The adhered oligomer diffuses to the surface of container and concentrates thereon during forming even if oligomer amount in the resin is small enough, and it moves on the mold surface to produce a crystalline buildup, which is known as a peculiar phenomenon in polyester resin container.
In addition, as the aforementioned mirror finished metal mold for forming tends to have poor releasability, the surface of metal mold for forming is roughened, which possibly produces a container with a roughened surface.
In particular, in case of forming a heat resistant polyester resin container, the metal mold is heated to a temperature range of 100 to 180° C. to heat set the container for the purpose of providing heat resistance. Therefore, following problems are caused; viscosity of the polyester resin increases, releasing force from the metal mold surface increases, and containers tend to be distorted when they are released from the metal mold.
The aforementioned critical problems in this industrial field should be solved for the reason that they deteriorate the production efficiency and the product quality in the high velocity mass production of polyester resin container in the present time.
Meanwhile, patent document 1 and 2 propose a metal mold for forming plastic of which surface is coated with fluoropolymer resin to form a releasing membrane. Patent document 3 proposes adhering a foundation membrane on a metal mold for forming and a releasing membrane consisting of an amorphous fluoropolymer resin coating on the foundation membrane, and removing and then reproducing the releasing membrane by cleaning and dissolving off the amorphous fluoropolymer resin coating in an inert fluoropolymer liquid, followed by coating an amorphous fluoropolymer resin dissolved in an inert fluoropolymer liquid on the releasing membrane, then drying and heating.
Applicant of the present invention also proposes a metal mold for polyester resin container of which surface is coated with a fluoropolymer resin to solve the aforementioned problems during forming polyester resin container in patent document 4 and 5.    PATENT DOCUMENT 1: Publication No. Hei4-353406    PATENT DOCUMENT 2: Publication No. Hei4-353407    PATENT DOCUMENT 3: Publication No. Hei5-245848    PATENT DOCUMENT 4: Publication No. 2002-18858    PATENT DOCUMENT 5: Publication No. 2003-393189